I have heard that the radiators can develop an internal crack allowing
coolant to bypass the core. Looking from underneath you can see that both
input and output are on the same side of the radiator. 
To test it you probably need to remove it though. Either a visual
inspection through the openings or hook a hose up to it from your hot
water heater. As water passes check both sides of the core for cold
spots.
Good Luck & Drive Safely
Ken Lewis
Kernersville NC <Heart of the Triad>
86 VW Transporter Three Door Pickup (168K)
60 Porsche T5 Coupe